Mr Back stepped down with immediate effect but will remain on the payroll until Aug 15. He will collect a £450,000 cash pay-off instead of working his notice period, with extra historical share awards taking the 66-year-old's golden parachute to £1m. However, he will miss out on about £2.7m of other bonuses.

The German-born boss had been had been criticised for running the company from his Swiss penthouse, while postal workers brave the risk of coronavirus to deliver the mail.

His departure comes just two weeks after billionaire Daniel Kretinsky - known as the “Czech sphinx” because of his inscrutable nature - revealed he had built up a 5pc stake in the FTSE 250 firm, making him one of its biggest investors.

Rico Back is leaving Royal Mail
Credit: Daniel Bockwoldt

Royal Mail declined to comment on the finer details over the timing of Mr Back’s exit.

It follows months of industrial unrest, with unions claiming he had abandoned promises made by predecessor Dame Moya Greene.

Staff were furious over plans for sweeping changes to working practices, leaving the company on the brink of its first strikes in a decade and potentially triggering severe disruption for millions of British families and businesses.

Mr Back, who previously worked for Royal Mail's overseas arm GLS, had sought to drag the company's 130,000-strong heavily unionised workforce into the 21st century with plans to increase automation. He wanted the postal service to better challenge more nimble rivals such as Amazon.

He has also disappointed investors, including the hundreds of thousands of ordinary savers who bought in when the firm was privatised in 2013.

Mr Back has overseen a steep fall in Royal Mail’s share price, its ejection from the blue-chip FTSE 100 and more recently the cancellation of of the company’s prized dividend.

Shares jumped more than 8pc to 175.9p, valuing the company at just under £1.8bn.

Terry Pullinger, deputy general secretary of the Communication Workers Union, said Mr Back’s exit was not a surprise. “Under his leadership this great public service has gone to an all-time low. Hopefully now with a new leadership with the right values, we can protect this great British institution for many years to come.”

Mr Williams will be supported by interim chief executive Stuart Simpson as he grapples to save the company's fortunes. There are no plans to backtrack on the current strategy, sources said.

The chairman is likely to face concerns that he is spread too thinly.

Mr Williams is overseeing the biggest review of Britain’s railways since the Nineties for the Government. He is also the deputy chairman of struggling retailer of John Lewis - which has been hammered by the coronavirus crisis along with most of the high street - and chairman of Halfords.

Royal Mail was facing a faster-than-expected slump in letter revenue even before the Covid-19 catastrophe hit.

A rise in parcel deliveries over the last month has failed to offset a fresh collapse in letter deliveries, which plunged 33pc during April.

The firm has also been forced to foot a £40m bill to cover the costs of overtime and employing agency workers due to high levels of absence, amid fears of poor staff morale.

The company was forced to suspend Saturday deliveries, with up to a fifth of employees off in the past few weeks.

Mr Back said: “It has been a privilege to lead a company that is so much a part of UK life at this crucial time in its history.

“I am proud of what I, together with our dedicated and loyal team, helped to build in Royal Mail and GLS. I look forward to seeing Royal Mail transform into a parcels-led, international delivery company, that continues to touch the lives of millions across the world.”